This has been stated ad nauseum, but I'm going to reiterate it.
It's the kids' damn faults.
Surely not all of them thought it was a great idea, but take the perspective of a teenager about to graduate high school. "What's the worst that can happen? We're having fun with friends at a water park. Everybody does this every year." I mean, it's not even like all 30 kids on the slide decided to do this. I bet most of them were just going along due to peer pressure.
I'm surprised that more of them weren't killed. A 40 foot fall is usually enough to kill someone, even if they execute a roll at the bottom. Falling that length with an unsafe landing, even if you hit other parts of the slide on the way down, would surely kill all of them, I would assume.